Biochemistry Chapter 24

A gene:

• encodes an enzyme.

• that is 1500 bp of genomic DNA will produce a protein 500 amino acids in length.

• is all the DNA that encodes the sequence of some protein or RNA product.

• can never be a cause of phenotype.

Answer explanation

The correct answer states that a gene is all the DNA that encodes the sequence of some protein or RNA product. This definition encompasses both protein-coding and non-coding genes, making it the most accurate choice.

The major problem associated with intracellular packaging of DNA is:

• that the DNA has no organization to its arrangement in the cell.

• the fact that the total length of DNA in a cell is many times longer than the dimensions of the cell.

• avoided by having some of the DNA in the form of plasmids.

• the rigid nature of DNA structure.

Answer explanation

The correct choice highlights that the total length of DNA in a cell exceeds the cell's dimensions, making packaging a significant challenge. This necessitates efficient organization to fit the DNA within the limited space.

Which chromosomal element occupies the SMALLEST relative proportion of DNA in the human genome?

• highly repetitive sequences

• satellite DNA

• exons

• genes

• introns

Answer explanation

Exons, which are coding regions of genes, represent a small fraction of the human genome compared to highly repetitive sequences, satellite DNA, genes, and introns, making them the smallest relative proportion of DNA.

What is the functional consequence of underfunctioning telomerases?

• nothing, as telomeres contain no functional information

• more poly(A)-tail regions at the telomeres

• severe shortening of chromosomes and eventual truncation/loss of functional genes

• loss of trans-acting elements

• protection against cancer

Answer explanation

Underfunctioning telomerases lead to severe shortening of chromosomes, which can result in the truncation or loss of functional genes. This is critical as telomeres protect chromosome ends from degradation.

Which statement regarding chromosomes is false?

• They are all of natural origin.

• They are circular in bacteria.

• They contain centromeres in eukaryotes.

• They contain telomeres in eukaryotes.

Answer explanation

The statement "They are all of natural origin" is false because some chromosomes can be artificially created or modified in laboratories, unlike the other statements which accurately describe chromosome characteristics.

Which statement about the human genome is false?

• A human somatic cell has 46 chromosomes in all.

• The human genome contains more than 20,000 protein-coding genes.

• Human cells contain more DNA than the cells of any other organism.

• Each human chromosome contains a single, very large, duplex DNA molecule.

Answer explanation

The statement 'Human cells contain more DNA than the cells of any other organism' is false. While humans have a complex genome, some organisms, like plants, can have more DNA due to polyploidy.

Which statement about mitochondrial DNA (mtDNA) is false?

• mtDNA has an evolutionary origin in the chromosomes of ancient bacteria that gained access to the cytoplasm of host cells.

• mtDNA codes for tRNAs, rRNAs, and a few proteins.

• Each mitochondrion contains a single copy of the mtDNA.

• mtDNA is much smaller than nuclear chromosomes.

Answer explanation

The statement 'Each mitochondrion contains a single copy of the mtDNA' is false. In fact, each mitochondrion can contain multiple copies of mtDNA, allowing for a higher number of mitochondrial genomes within a cell.

Supercoiling:

• is the coiling of antiparallel single strands of DNA around each other in the Watson-Crick model of DNA structure.

• can be thought of as the coiling of the central axis of a DNA double helix upon itself.

• is the overwinding of DNA, and relaxation is the underwinding of DNA.

• makes it more difficult to separate the two strands of a DNA double helix.

Answer explanation

The correct choice describes supercoiling as the coiling of the DNA double helix's central axis upon itself, which accurately captures the concept of supercoiling in DNA structure.

The finding that DNA is commonly underwound was a surprise. Predictions based on the B form of closed, circular DNA suggest that underwinding serves what purpose?

• It facilitates supercoiling.

• It prevents strain on the helix and strand breaks.

• It promotes more rapid rates of endonuclease activity.

• It prevents supercoiling and promotes spontaneous strand separation.

• It tends to prevent common structural mutations such as thymine dimers.

Answer explanation

Underwinding of DNA facilitates supercoiling, which is essential for compacting the DNA structure and regulating access to genetic information. This is crucial for processes like replication and transcription.

If a negatively supercoiled (ΔLk = –32) closed circular DNA of 2520 bp is relaxed by one catalytic cycle by a type II topoisomerase, what is the resulting Lk?

• 240

• 242

• 270

• 210

Answer explanation

The initial Lk is 2520 bp - 32 = 2488. Relaxing by one cycle (removing 2 from Lk) gives 2488 - 2 = 2486. However, for negatively supercoiled DNA, the effective Lk is 210 after accounting for the supercoiling. Thus, the resulting Lk is 210.